Covalent attachment of the hydrophilic polymer, poly(ethylene glycol), abbreviated “PEG,” to molecules and surfaces is of considerable utility in areas such as biotechnology and medicine. PEG is a polymer that possesses many beneficial properties. For instance, PEG is soluble in water and in many organic solvents, is non-toxic and non-immunogenic, and when attached to a surface, PEG provides a biocompatible, protective coating. Common applications or uses of PEG include (i) covalent attachment to proteins to, for example, extend plasma half-life and reduce clearance through the kidney, (ii) attachment to surfaces such as in arterial replacements, blood contacting devices, and biosensors, (iii) use as a soluble carrier for biopolymer synthesis, and (iv) use as a reagent in the preparation of hydrogels.
In many if not all of the uses noted above, it is necessary to first activate the PEG by converting its hydroxyl terminus to a functional group capable of readily reacting with a functional group found within a desired target molecule or surface, such as a functional group found on the surface of a protein. For proteins, typical functional groups include functional groups associated with the side chains of lysine, cysteine, histidine, arginine, aspartic acid, glutamic acid, serine, threonine, and tyrosine, as well as the N-terminal amino functional group and the C-terminal carboxylic acid functional group.
The PEG used as a starting material for most PEG activation reactions is typically an end-capped PEG. An end-capped PEG is one where one or more of the hydroxyl groups, typically located at a terminus of the polymer, is converted into a non-reactive group, such as a methoxy, ethoxy, or benzyloxy group. Most commonly used is methoxyPEG, abbreviated as mPEG. End-capped PEGs such as mPEG are generally preferred, since such end-capped PEGs are typically more resistant to cross-linking and aggregation. The structures of two commonly employed end-capped PEG alcohols, mPEG and monobenzyl PEG (otherwise known as bPEG), are shown below,
wherein n typically ranges from about 10 to about 2,000.
Despite many successes, conjugation of a polymer to an active agent is often challenging. For example, it is known that attaching a relatively long poly(ethylene glycol) molecule to an active agent typically imparts greater water solubility than attaching a shorter poly(ethylene glycol) molecule. One of the drawbacks of some conjugates bearing such long poly(ethylene glycol) moieties, however, is the possibility that such conjugates may be substantially inactive in vivo. It has been hypothesized that these conjugates are substantially inactive due to the length of the poly(ethylene glycol) chain, which effectively “wraps” itself around the entire active agent, thereby limiting access to ligands required for pharmacologic activity.
The challenge associated with relatively inactive conjugates bearing relatively large poly(ethylene glycol) moieties has been solved, in part, by using “branched” forms of a polymer conjugated to the active agent. Examples of a branched version of a poly(ethylene glycol) derivative are conventionally referred to as “mPEG2-N-hydroxysuccinimide” and “mPEG2-aldehyde” as shown below:
wherein n represents the number of repeating ethylene oxide monomer units. Other branched polymer structures comprise a polyol core, such as a glycerol oligomer, having multiple polymer arms covalently attached thereto at the sites of the hydroxyl groups. Exemplary branched polymer structures having a polyol core are described in U.S. Pat. No. 6,730,334.
Another reason for using branched structures like those above in the synthesis of a conjugate relates to the desire to increase the in vivo circulation time of the drug. Larger polymers are known to have longer circulation times than smaller polymers. Hence, drugs attached to higher molecular weight polymers have longer circulation times, thus reducing the dosing frequency of the drug, which must often be injected.
Although addressing some of the shortcomings associated with relatively large polymer sizes, branched polymer structures have been associated with drawbacks of their own. For example, although a branched polymer attached to an active agent may have satisfactory pharmacologic activity, a branched polymer can still suffer from insufficient clearance from the body. Thus, while it is desirable to increase the circulation time of a drug by forming a drug-polymer conjugate, there is a competing desire to ensure that the conjugate remains susceptible to elimination from the body.
As a result, there is an ongoing need in the art for linear, branched, or multiarm polymer derivatives that have the molecular weight necessary to provide a for a conjugate that has the desirable in vivo circulation time, but which also exhibits timely clearance from the body. The present invention addresses this and other needs in the art.